The present invention relates to the field of the prevention against explosion of electrical transformers cooled by a large volume of combustible fluid.
Electrical transformers exhibit losses both in the windings and in the core, for which reason the heat produced needs to be dissipated. High-power transformers are thus generally cooled using a fluid such as oil. The oils used are dielectric and can ignite above a temperature of the order of 140xc2x0 C. Since transformers are very expensive elements, particular attention must be paid to protecting them.
An insulation fault first generates a strong electric arc which prompts action by the electrical protection systems, which trip the supply relay of the transformer (circuit breaker). The electric arc also causes consequent dissipation of energy, which generates release of gas from decomposition of the dielectric oil, in particular hydrogen and acetylene.
After the gas has been released, the pressure inside the enclosure of the transformer increases very rapidly, whence an often very violent deflagration. The deflagration results in extensive tearing of the mechanical connections in the enclosure (bolts, welds) of the transformer, which brings the said gases into contact with the oxygen in the surrounding air. Since acetylene can spontaneously ignite in the presence of oxygen, combustion immediately starts and causes the fire to spread to other on-site equipment which may also contain large quantities of combustible products.
Explosions are due to short-circuits caused by overloads, voltage surges, progressive deterioration of the insulation, and insufficient oil level, the appearance of water or moisture or the failure of an insulating component.
Fire protection systems for electrical transformers are known in the prior art, and these are actuated by combustion or fire detectors. However, these systems are implemented with a significant time lag, when the oil of the transformer is already burning. It then being necessary to make to with limiting the combustion to the equipment in question, and to prevent the fire from spreading to the neighbouring plant.
In order to slow down the decomposition of the dielectric fluid due to an electric arc, silicone oils may be used instead of conventional mineral oils. However, explosion of the enclosure of the transformer due to the increase in the internal pressure is delayed only by an extremely short time, of the order of a few milliseconds. This length of time does make it possible to engage means which can prevent the explosion.
The document WO-A-97/12379 discloses a method for prevention against explosion and fire in an electrical transformer provided with an enclosure filled with combustible coolant, by detecting a break in the electrical insulation of the transformer using a pressure sensor, depressurizing the coolant contained in the enclosure, using a valve, and cooling the hot parts of the coolant by injecting a pressurized inert gas into the bottom of the enclosure in order to stir the said coolant and prevent the oxygen from entering the enclosure of the transformer. This method is satisfactory and makes it possible to prevent the enclosure of the transformer from exploding.
The object of the present invention is to provide an improved device allowing extremely rapid decompression of the enclosure in order to further increase the probability of safeguarding the integrity of the transformer, of the on-load tap changers and of the feed-throughs.
The device for prevention against explosion according to the invention is intended for an electrical transformer comprising an enclosure filled with combustible coolant, and a means for decompressing the enclosure of the transformer. The decompression means comprises a rupture element provided with a retention part including first zones which have a reduced thickness in comparison with the rest of the retention part and are capable of tearing without fragmenting when the said element ruptures, and second zones which have reduced thickness in comparison with the rest of the retention part and are capable of folding without tearing when the said element ruptures. The said rupture element is capable of breaking when the pressure inside the enclosure exceeds a predetermined ceiling.
Preferably, the rupture element is provided with a sealing component which is arranged on the coolant side and is capable of closing off small-diameter holes formed in the retention part. The holes may form tear initiators and be adjacent to the first zones of reduced thickness.
In one embodiment of the invention, the sealing component is in the form of a lining on the retention part, the said lining being preferably based on polytetrafluoroethylene.
Preferably, the retention part has a domed shape with convexity outwards, on the opposite side from the coolant.
In one embodiment of the invention, the retention part is metallic, made of stainless steel, aluminium or aluminium alloy.
Preferably, the device comprises a rupture-detection means integrated with the rupture element, which makes it possible to detect the pressure in the enclosure relative to the predetermined ceiling.
In one embodiment of the invention, the rupture-detection element comprises an electrical wire capable of breaking at the same time as the rupture element.
In one embodiment of the invention, the electrical wire is adhesively bonded on the rupture element.
Advantageously, the electrical wire is arranged on the opposite side of the retention part to the coolant.
In one embodiment of the invention, the electrical wire is covered with a protective film.
The invention also relates to a system for prevention against explosion of an electrical transformer comprising an enclosure filled with combustible coolant, and a means for decompressing the enclosure of the transformer. The system comprises a plurality of devices as described above, including one or more on a main enclosure containing the windings and one on each on-load tap changer.
The system may comprise at least one device as described above, on at least one electrical feed-through.
Simultaneously, the rupture element ruptures with the result that the enclosure becomes decompressed, and the wire ruptures with the result that an excessive and abnormal pressure is detected.
Of course, terms such as xe2x80x9con the fluid sidexe2x80x9d or xe2x80x9con the opposite side from the fluidxe2x80x9d refer to the situation before rupture.
The device for prevention against explosion is designed for the main enclosure of a transformer, for the enclosure of the on-load tap changer or changers, and for the enclosure of the electrical feed-throughs, the latter enclosure also being referred to as the oil box. The purpose of the electrical feed-throughs is to isolate the main enclosure of a transformer from the high- and low-voltage lines to which the windings of the transformer are connected by means of the output rods. Each output rod is surrounded by an oil box containing a certain quantity of insulating fluid. The fluid for insulating the feed-throughs and/or oil boxes is an different oil from that of the transformer.
A nitrogen injection means may be provided which is connected to an upper part of an oil box and can be triggered when a fault is detected. Injecting nitrogen may promote the discharge of the fluid downstream of the rupture element. Injecting nitrogen may above all prevent air from entering the oil box, entry of air being capable of promoting combustion.
The device for prevention against explosion may be provided with a means for detecting the tripping of the supply relay of the transformer and with a control unit which receives the signals output by the sensor means of the transformer and which is capable of emitting control signals.
The device for prevention against explosion may comprise a means for cooling the hot parts of the fluid, by injecting inert gas into the bottom of the main enclosure, which means is controlled by a control signal from a control unit. The reason for this is that some parts of the coolant undergo heating which can cause it to ignite. Injecting an inert gas at the lower part of the enclosure causes stirring of the coolant, which equilibrates the temperature and reduces the release of gas.